There are two problems in computer science, accepting payments and naming things.

Reddit's principal problem is that the first person to take r/foo is often a BDFL for foo for life, and no other subreddit about foo will ever be quite as recognizable. If we instead had subreddits with a numeric ID and a non-unique display name, that problem would be solved.

Payments would also solve the spam problem, but many users who have $1 can't easily get that $1 to Reddit, so that's not really an option either.